The "handling characteristics" of a vehicle's tire are of paramount important in a "high performance" tire. By "handling characteristics" we refer to a combination of individually categorized characteristics referred to in greater detail hereinafter, which are not readily observed by an average driver in an average automobile at legal highway speeds in this country. Such handling characteristics are deliberately designed for a specific purpose, including for off-the-highway high speed driving, as on a race track, or for law enforcement patrol cars, and categorically not designed for use on the average highway under normal circumstances.
It was desired to provide improved handling characteristics without noticeable loss in wet and dry traction, and treadwear resistance. Since improvement in one of the foregoing properties generally results in a concomitant denigration of at least one of the others, this long-sought-after goal in a high performance tire seemed unattainable.
This invention is related to a high performance pneumatic, tubeless radial tire with long-sought-for characteristics, and more particularly to the tread composition for such a tire.
It is well known that high surface area, small particle size blacks provide greater reinforcement, as evidenced by improved resistance to abrasion, tensile strength and tear properties; concomitantly, superfine blacks also provide higher hysteresis and poorer dynamic performance attributable to high heat build-up, both adverse properties. In addition it is known that larger size blacks provide reduced rolling resistance and poorer traction, both wet and dry. This combination of adverse properties, namely high heat build-up and poorer dynamic performance resulting in increased rolling resistance has never been correlated to the handling characteristics of a tire. More importantly, it was not known that surface activity, which refers generally to the manner in which the black interacts with different compounding ingredients, was closely related to "handling", traction and abrasion (wear) resistance.
In the recent past, the emphasis has been on reduction of rolling loss for lower fuel consumption without sacrificing wear resistance. Ignoring changes in tire construction to aid in this respect, several approaches have been taken towards attaining this goal, as taught in U.S. Pat. Nos. 3,824,206; 4,224,197; and 4,281,703, inter alia; and United Kingdom patent applications Nos. GB 2,082,486A and 2,057,455A. None was concerned with deliberately increasing the rolling resistance and heat buildup in a tire, which is what we have done.
In currently marketed predominantly styrene-butadiene copolymer rubber ("SBR") tires, carbon black is the essential rubber reinforcing ingredient. As such a wide variety of blacks are used in the treads of pneumatic radial and other tires, but a black also provides essential traction, as do other reinforcing ingredients such as silica; but there is no known way to predict the effects of the use of a particular black, which effects must therefore be determined by experiment. The choice of the type and amount of carbon black in the tread compound influences many performance properties of the tire.
There are about seven main classifications of carbon black, including in decreasing order of market share, but not of importance, the following: HAF (N300); GPF (N600); FEF (N500); SRF (N700); ISAF (N200); Thermal; and SAF (N100). This invention is related to the use of SAF type blacks which currently constitute about 10% of the carbon black market for tires. These N100 blacks are currently used in tires where excellent abrasion resistance and tear properties are of paramount importance, as for example in off-the-road tires for all types of vehicles.
The distinction between grades of carbon blacks are based on three main factors which are broadly classified as follows:
1. Particle size, particularly as it relates to surface area.
2. Structure, a general measure of particle-to-particle association.
3. Chemical composition of the surface, or surface activity.
Surface area, generally measured by iodine number ("I.sub.2 No."), and structure, generally a measure of void volume, in turn measured by dibutyl phthalate absorption ("DBPA"), are key attributes which characterize a carbon black.
This invention relates to the exploitation of a particular property of a SAF carbon black having high structure and a surface area with a unique level of volatiles, identified with a relatively lower I.sub.2 No. than other commercially available SAF blacks. Such blacks, when substituted for other blacks conventionally used in tread compounds usually used, was not expected to provide such an outstanding combination of desirable properties, particularly handling and traction, as we found by experimentation with numerous tread compounds.
A detailed discussion of some of the foregoing considerations is presented in an article titled "The Effects of Carbon Black and Other Compounding Variables on Tire Rolling Resistance and Traction" by W. M. Hess and W. K. Klamp, Rubber Chemistry and Technology, Vol. 56, No. 2 May-June 1983. A predominantly SBR tire with minor amounts of polybutadiene rubber (BR) essentially all of which is cis-, and/or natural rubber (NR), and an oil loading of no more than 70/40 black/oil ratio with a high structure N220 black, was used to study the effects of carbon black and other variables on tire rolling resistance and traction, among other objectives. The blacks tested varied in nitrogen surface area (N.sub.2 SA), ASTM tint, and DBPA.
It has also been recognized that the physico-chemical nature of the carbon black particles' surface, and particularly, the nature of the carbon atoms at the surface of a particle, may affect rubber reinforcement. Similarly the chemical nature of the particles' surface, and particularly the presence of oxygen at the surface, along with phenolic, ketonic and carboxylic groups, inter alia which are known to be present on the surface, affect the crosslinking of the rubber and its vulcanized properties. See Rubber Technology and Manufacture, edited by C. M. Blow, p 180, CRC Press, International Scientific Series (1971). But there appears to be no direct connection between chemical nature of the surface (i.e. the individual chemical groups of the particle surface) and the properties which the black confers on the rubber. (see Blow, supra at pg 186).
Experimental results with N234, N251, N375 and N220 blacks showed that an increasing rolling loss coefficient caused by increasing the nitrogen surface area of the black gave a correlation coefficient of 0.97. A similar statement may be made regarding the I.sub.2 No. and tinting strength of blacks. No such meaningful correlation was found with compressed DBP values of the blacks. Dry traction results showed a poor correlation with rolling loss, but did correlate well with I.sub.2 No. and tinting strength. (see "The Effects of Carbon Black on Rolling Resistance of a Tire Tread Compound" by J. R. West et al. Rubber Chemistry and Technology, supra, at pg 509).
Not surprisingly, there is no indication in either of the immediately foregoing references that surface activity of the blacks tested may have any noteworthy or desirable effects with respect to a combination of traction, abrasion resitance and handling, despite the expected high rolling loss of a tire having a tread made from a high structure black with a superfine particle size and a relatively contaminated surface (indicated by a relatively lower I.sub.2 No.). More particularly, there was no indication that the "volatiles" or contamination on the surface (as measured by I.sub.2 No.) of a high surface area, high structure black, might be of great significance with respect to providing excellent handling, traction and abrasion resistance.